1. Field of the Invention
This invention relates to thermoset and polyareneazole pulp for use as a reinforcement material in products including for example fluid seals and friction materials, as a processing aid including its use as a thixotrope, and as a filter material. The invention further relates to processes for making such pulp.
2. Description of Related Art
Fibrous and non fibrous reinforcement materials have been used for many years in friction products, fluid sealing products and other plastic or rubber products. Such reinforcement materials typically must exhibit high wear and heat resistance.
Asbestos fibers have historically been used as reinforcement materials, but due to their health risks, replacements have been made or proposed. However, many of these replacements do not perform as well as asbestos in one way or another.
Research Disclosure 74-75, published February 1980, discloses the manufacture of pulp made from fibrillated KEVLAR® brand para-aramid fibers of variable lengths and use of such pulp as a reinforcement material in various applications. This publication discloses that pulp made from KEVLAR® brand para-aramid fibers can be used in sheet products alone, or in combination with fibers of other materials, such as NOMEX® brand meta-aramid, wood pulp, cotton and other natural cellulosics, rayon, polyester, polyolefin, nylon, polytetrafluoroethylene, asbestos and other minerals, fiberglass and other, ceramics, steel and other metals, and carbon. The publication also discloses the use of pulp from KEVLAR® brand para-aramid fiber alone, or with KEVLAR® brand para-aramid short staple, in friction materials to replace a fraction of the asbestos volume, with the remainder of the asbestos volume being replaced by fillers or other fibers.
U.S. Patent Application Publication 2003/0022961 (to Kusaka et al.) discloses friction materials made from a friction modifier, a binder and a fibrous reinforcement made of a mixture of (a) a dry aramid pulp and (b) wet aramid pulp, wood pulp or acrylic fiber pulp. Dry aramid pulp is defined as an aramid pulp obtained by “the dry fibrillation method”. The dry fibrillation method is dry milling the aramid fibers between a rotary cutter and a screen to prepare the pulp. Wet aramid pulp is defined as an aramid pulp obtained by “the wet fibrillation method”. The wet fibrillation method is milling short aramid fibers in water between two rotary discs to form fibrillated fibers and then dehydrating the fibrillated fibers, i.e., the pulp. Kusaka et al further disclose a method of mix-fibrillating fibers by first mixing plural types of organic fibers that fibrillate at a definite ratio, and then fibrillating the mixture to produce a pulp.
Polypyridobisimidazole polymer is a rigid rod polymer. Fiber made from this polymer (such as the polymer composition of which is referred to as PIPD and is known as the polymer used to make M5® fiber) is known to be useful in both cut and flame resistant protective apparel. Rigid-rod polymer fibers having strong hydrogen bonds between polymer chains, e.g., polypyridobisimidazoles, have been described in U.S. Pat. No. 5,674,969 to Sikkema et al. An example of a polypyridobisimidazole is poly(1,4-(2,5-dihydroxy)phenylene-2,6-pyrido[2,3-d:5,6-d′]bisimidazole), which can be prepared by the condensation polymerization of tetraaminopyridine and 2,5-dihydroxyterephthalic acid in polyphosphoric acid. Sikkema discloses that pulp can be made from these fibers. Sikkema also describes that in making one- or two-dimensional objects, such as fibers, films, tapes, and the like, it is desired that polypyridobisimidazoles have a high molecular weight corresponding to a relative viscosity (“Vrel” or “hrel”) of at least about 3.5, preferably at least about 5, and more particularly equal to or higher than about 10, when measured at a polymer concentration of 0.25 g/dl in methane sulfonic acid at 25° C. Sikkema also discloses that good fiber spinning results are obtained with poly[pyridobisimidazole-2,6-diyl(2,5-dihydroxy-p-phenylene)] having relative viscosities greater than about 12, and that relative viscosities of over 50 (corresponding to inherent viscosities greater than about 15.6 dl/g) can be achieved.
There is an ongoing need to provide alternative pulps that both perform well in products and that are low in cost. Despite the numerous disclosures proposing lower cost alternative reinforcement materials, many of these proposed products do not adequately perform in use, cost significantly more than currently commercial products, or have other negative attributes. As such, there remains a need for reinforcement materials that exhibit high wear and heat resistance, and that are comparable or less expensive than other commercially available reinforcement materials.